An antenna is an electromagnetic (EM) device configured to cause an EM wave to transition between being propagated in a confined medium and being propagated in an unconfined medium. The confined medium can be, for example, a transmission line, a waveguide, a coaxial cable, the like, or any combination of these. The unconfined medium can be, for example, air, space, or a combination of these. An antenna is used to receive an EM wave from being propagated in an unconfined medium to being propagated in a confined medium. Conversely, an antenna is used to transmit an EM wave from being propagated in a confined medium to being propagated in an unconfined medium. Antennas are essential components of equipment used in systems configured to exploit the propagation of EM waves in unconfined media. Such systems can include, for example, broadcast radio, broadcast television, two-way radio, radar, mobile telephones, cellular communications, satellite communications, wireless computer networks, wireless personal area networks, space-based navigation systems, radio astronomy, and the like.
A resonant antenna is designed to produce a standing EM wave, related to a specific wavelength (or range of wavelengths), between two terminals of the antenna. An antenna element of a resonant antenna can have a length equal to one quarter of the specific wavelength. However, a standing EM wave can also be produced when the length of the antenna element is shorter than one quarter of the specific wavelength by increasing the capacitive reactance of the impedance of the antenna. Similarly, a standing EM wave can also be produced when the length of the antenna element is longer than one quarter of the specific wavelength by increasing the inductive reactance of the impedance of the antenna. In this manner, an electrical length of an antenna element can be longer than a physical length of the antenna element when a standing EM wave can also be produced when the physical length of the antenna element is shorter than one quarter of the specific wavelength. Likewise, a standing EM wave can also be produced when the length of the antenna element is longer than one quarter of the specific wavelength by decreasing the capacitive reactance of the impedance of the antenna. Similarly, a standing EM wave can also be produced when the length of the antenna element is shorter than one quarter of the specific wavelength by decreasing the inductive reactance of the impedance of the antenna. In this manner, an electrical length of an antenna element can be shorter than a physical length of the antenna element when a standing EM wave can also be produced when the physical length of the antenna element is longer than one quarter of the specific wavelength.
A dipole antenna is a common form of a resonant antenna. A dipole antenna includes two antenna elements typically disposed symmetrically with respect to each other. When a dipole antenna is used to transmit an EM wave, an electrical signal, characterized by a specific wavelength, is provided in a balanced manner to the interior terminals of each of the antenna elements. The electrical signal in this case is represented by the oscillation of equal but opposite currents. One of the currents is provided to the interior terminal of one of the antenna elements and the equal but opposite current is provided to the interior terminal of the other one of the antenna elements. Because the currents cannot flow beyond the exterior terminals of each of the antenna elements, the oscillation of the currents causes an oscillation of accumulated charges at the exterior terminals of each of the antenna elements, which in turn causes an oscillation of voltages between the exterior terminals of the antenna elements. The oscillation of the currents and the voltages causes a standing EM wave to be produced between the exterior terminals of the antenna elements. The standing EM wave is characterized by a measure of currents at the exterior terminals that is effectively zero and a measure of currents at the interior terminals that oscillates between a maximum value in one direction and an equal maximum value in the opposite direction. The standing EM wave is further characterized by a measure of voltages at the interior terminals that is effectively zero and a measure of voltages at the exterior terminals that oscillates between a maximum value and an equal but opposite maximum value.
A monopole antenna is another common form of a resonant antenna. A monopole antenna includes an antenna element typically disposed perpendicular to a ground plane. The ground plane typically is a conductive surface and ideally has an infinite length and an infinite width. To be effective, the ground plane should have a length at least equal to one quarter of the specific wavelength. The ground plane can be the actual ground (i.e., the earth), can be connected to the actual ground, or can be separated from the actual ground. When a monopole antenna is used to transmit an EM wave, an electrical signal, characterized by the specific wavelength, is provided in a balanced manner to the ground plane and to the terminal (of the antenna element) nearest to the ground plane. Because the current provided to the terminal nearest to the ground plane cannot flow beyond the terminal (of the antenna element) farthest from the ground plane, the oscillation of the current causes an oscillation of accumulated charges at the terminal farthest from the ground plane, which in turn causes an oscillation of voltages between the terminals of the antenna element. The oscillation of the currents and the voltages causes a standing EM wave to be produced between the terminals of the antenna element. The standing EM wave is characterized by a measure of current at the terminal farthest from the ground plane that is effectively zero and a measure of current at the terminal nearest to the ground plane that oscillates between a maximum value in one direction and an equal maximum value in the opposite direction. The standing EM wave is further characterized by a measure of voltage at the terminal nearest to the ground plane that is effectively zero and a measure of voltage at the terminal farthest from the ground plane that oscillates between a maximum value and an equal but opposite maximum value. The ground plane acts to reflect the EM wave that radiates from the antenna element. Ideally, the reflection of the EM wave is such that the radiation pattern from the monopole antenna resembles a radiation pattern that would be produced from a theoretical dipole antenna having both the antenna element of the monopole antenna and another antenna element symmetrically disposed on the other side of the ground plane.